1. Field of the Invention
The present invention relates to a technology of improving image quality of an output image in digital image processing, suitable in particular for use in digital copying machines, facsimile machines, etc.
2. Discussion of the Background
A dot-concentrated dither method has been known as a halftone processing technology having advantages in dot formation stability and graininess, and has been used in copying machines, printers, etc. having an electrophotographic printing engine. However, if a dither process is applied to an inputted halftone dot image, an interfering moire is caused by a periodicity of a dither matrix and that of the halftone dot image. Various methods have been proposed for suppressing such a moire.
For example, in an apparatus, a smoothing process is performed to an input image of an original manuscript having a halftone dot area to reduce occurrence of an interfering moire caused by undulation of density (cyclic repetition of a high level density and a low level density) in the halftone dot area, and thereafter an edge enhancement process is performed to keep sharpness of edge portions of the image. In another apparatus, occurrence of a moire in a halftone dot area is suppressed by using a halftone dot level calculating filter configured to respond to a halftone dot portion of an input image and by performing a smoothing process according to a result of calculation at the halftone level calculating filter.
In duplicating an original manuscript by a digital copying machine, a socalled backside information duplicating phenomenon occurs, wherein information or an image on a backside of the original manuscript transmits through a sheet of the original manuscript, so that the backside information or image is duplicated while being mixed with an image on a front (duplicating) side of the original manuscript.
Once such undesired information or an image on the backside of the original manuscript has been mixed with the desired image on the front (duplicating) side of the original manuscript and converted to image information, in the resultant image, the backside information or image has substantially the same density characteristic as a low-contrast portion of the desired image on the front (duplicating) side of the original manuscript. Therefore, it is relatively difficult to remove only the undesired backside information or image from the image information. In one method to prevent such a backside information duplicating phenomenon from occurring, so-called a γ conversion process the output value for a low-contrast portion of an image on a duplicating side of the original manuscript, at which input value the information or image on the backside of the original manuscript might be duplicated, is converted to zero representing “white”.
As described above, a smoothing process is performed to a halftone dot area to reduce the occurrence of an interfering moire. However, if a smoothing process is performed to a low-density area of the halftone dot area, the difference between a high level density and a low level density in the low-density area of the halftone dot area is eliminated, so that the density of the low-density area of the halftone dot area is uniformly converted to the low level density. Further, when performing a γ correction process after the smoothing process, to prevent a backside information duplicating phenomenon from occurring, a γ characteristic that converts a low-contrast portion to “white” is used. Therefore, the halftone dot area which has been processed by the smoothing process to be in a uniform low level density is converted to a further lower level density or to “white” (i.e., the output value is zero). As a result, the density of the low-density halftone dot area is reduced, or information of the low-density halftone dot area is omitted.